The only other possible source for ready-made brass I can suggest would be Bertram from Australia. IIRC, Widener's and Graf are/were the two major US outlets. It's usually quite pricey, but they have historically been our best source of cases for obsolete Brit/Euro cartridges.

If you must, or simply opt to, make them yourself I'd suggest consulting the Gun Digest "Handbook of Cartridge Conversions" or a similar reference for the dimensional specifications and drawings. Can't say for sure that those'll be easy to find given just how long this cartridge has been 'extinct', but surely they can be found somewhere with some digging. Another possibility might be the standard reference book "Cartridges of the World". There may even be some history and, perhaps, loading data there.

FWIW, making cases may well involve a lot more than simply shortening the 'parent' case. Be aware that many times the rim must be altered in thickness and/or circumference to work and at times that can be enough to render the resulting case head's primer pocket too shallow to allow proper seating without some further rather delicate modification.

Personally, I'd use a lot of oil and a swaged, pure lead ball of about 0.451" to slug the bore and either use a good calipher or find a machinist with a set of 'plug' gauges to determine your chamber mouth size. Those results will tell you whether you can use an available commercial mold/cast bullet or will need to resort to having one custom-made. IIRC, RCBS has a list of special-order molds for obsolete calibers so that could be worth checking, too. Neither of the latter options would come cheap though.